In wireless communication devices that use an RF power amplifier (PA) it is known that a severe Voltage Standing Wave Ratio (VSWR) load variation can temporarily or permanently degrade the performance of the power amplifier. To avoid this problem it is well-known in the art to provide an isolator between the output of the PA and an antenna that is fed by the PA. However, this approach requires the use of a separate element (the isolator), which can be large in size and which furthermore adds cost and complexity to the RF transmitter circuitry of which the PA forms a part.
In general, the operation of an RF PA is based on a tuned load line. The load line is defined by the bias voltage, bias current and load impedance. Correctly set, the load line optimizes the voltage and current waveforms of the PA output power transistor, as well as the desired output power where the optimum PA efficiency is achieved. An output-matching network is used to transform the output impedance, typically 50 ohms, to the optimum load impedance for the output transistor. The output-matching network is designed to provide the desired impedance transformation ratio with the desired output frequency.
In a typical radio transmitter the PA output is followed by a filter and/or a switch, and then by the antenna. The antenna is typically designed to have a 50 ohm input impedance when radiating into free space, and the filter and/or switch between the antenna and the PA is designed to have 50 ohm input and output impedances. Thus, in order to provide impedance matching the output impedance of the PA must also be 50 ohms. However, in portable, handheld communications devices the antenna impedance may vary from the ideal 50 ohm value due to the variability of the environment that the antenna radiates into. This change in the antenna impedance is seen as a mismatch by the output of the PA, and can result in a degradation in the RF transmitter performance.
A variation in the load impedance (i.e., the load mismatch) can present a serious problem in many systems, such as a wideband code division, multiple access (WCDMA) wireless communications system, since the WCDMA system requires that the PA exhibit good linearity. In the WCDMA system, a PA load mismatch can result in signal distortion and a degradation in performance. The end result is that under a load mismatch condition the stringent WCDMA transmitter requirements can become more difficult or even impossible to meet.
As was noted above, it is known in the art to use a matching device, such as an isolator or a circulator, to isolate the output of the PA from the antenna. By so doing the PA always sees a nominal 50-ohm impedance, despite the variation in antenna impedance, and signal distortion is avoided. A drawback of this approach is the additional loss that results in the matching device. Adding the matching device in the transmitter path also increases the component count, the cost, the power consumption and the required circuit board area. All of these factors are considered to be disadvantageous in handheld, battery powered, mass produced communications equipment, such as cellular telephones.
It is also known in the art to use a reflectometer and associated circuitry to protect the PA from severe load mismatches. The reflectometer is realized by a dual directional coupler having a length of λ/4 and associated circuitry. This technique uses the forward and reflected voltages to detect the VSWR mismatch, and associated circuitry is then used to compensate the power amplifier or surrounding circuitry to adapt to the VSWR mismatch. As with the use of the matching device, this technique increases the losses, cost and circuit board area of the transmitter.
It is also possible to detect the impedance mismatch condition, and then adapt an impedance matching network, or the PA, to compensate for the detected load mismatch condition. The adaptation can be made by adjusting the drive to the PA, adjusting the PA biasing, or to use a diode to transform the impedance to a more suitable form.
U.S. Pat. No. 5,564,086 describes one such conventional technique to protect a PA. Referring to FIG. 1, the output of the PA 1 is connected to a variable matching network 2, which in turn is connected to an antenna 4 via a directional coupler 3. The directional coupler detects a mismatch condition in combination with a diode detector, and provides output signals to a processor 5 that adjusts the variable matching network 2 accordingly. The directional coupler 3 can be used to sample the reflected signal 6A independently of the forward signal 6B, and the magnitude of the impedance presented by the antenna 4 can thus be determined. As can be appreciated, this approach involves adding circuitry and complexity to the RF transmitter, and thus suffers from disadvantages discussed above.
It is also known to use a current detector to sense PA power, such as is shown in U.S. Pat. Nos. 5,404,585, 5,448,770 and 4,859,967. It is also known to employ voltage detectors in conjunction with a PA, as described in U.S. Pat. Nos. 6,265,939, 6,002,922, 5,873,029, 5,659,253 and 4,041,410. It is further known to use an RF detector to control the PA, as described in U.S. Pat. Nos. 5,724,003, 5,432,473, 5,291,150, 5,278,994, 5,150,075, 4,709,403, 4,673,886 and 5,564,086.
It is further known that PA systems can have power detection and control, as shown in U.S. Pat. Nos. 6,188,277, 5,404,114, 5,003,271, 4,859,967, 4,727,337 and 4,122,400.
Further in this regard reference can also be made to U.S. Pat. No. 5,070,309, where an RF power amplifier has an On and an Off condition. When On, the PA normally receives and amplifies an input RF signal to provide an amplified output RF signal. Both RF current and RF voltage in an output circuit are detected. A first signal is provided from the RF current and exhibits a fixed amplitude, but a frequency and phase corresponding with that of the RF current. A second signal of fixed amplitude is provided that corresponds in frequency and phase with the RF voltage. The first and second signals are compared to provide a phase signal having a magnitude dependent upon the phase difference between the first and second signals. The phase signal is then compared with a reference and, based on the result of the comparison, the RF PA can be placed into the Off condition.
U.S. Pat. No. 6,289,205 B1 describes a high frequency power amplifier that includes an output stage for outputting an amplified high frequency signal, and measuring devices for measuring the power of the high frequency signal.
The foregoing U.S. patents do not fully address the PA problems that can result from VSWR load mismatches.